Add e2e smoke tests

[sunker]

What this PR does:

• configures Playwright and grafana/plugin-e2e according to these instructions
• adds simple e2e tests that verifies that the config editor, the query editor, the annotation editor and the variable query editor renders
• adds a workflow (copied from this article) that runs the smoke tests in all versions of Grafana that the plugin supports (decided by the grafanaDependency specified in package.json)

Note that the tests in this PR only verifies that the modules used in these frontend components loads and renders in the different Grafana runtimes. It does not verify that the data query pipelines work. To find inspiration on how to test data pipelines, refer to the docs.